I don't think all of you who were raised in the nonsensical subculture that buys into the idea of "lovers being more than friends and family" or uses the phrase "just a friend" as a way to diminish a social relationship and to depict that you don't care that deeply for them, have any right to be lecturing about emotional loyalty and respecting feelings in social relationships.
The seme is not problematic at all. He doesn't owe it to any random person to love them or be devoted to them just because he has good sexual chemistry with them. The only people you owe love and loyalty to is family. Everyone else is always a choice. You don't owe "romantic" attachment to everyone you have sex with just like you don't owe friendship to everyone you hang out with. Not feeling affection for random people doesn't make you problematic. He wasn't leading them on, he wasn't using whatever affection his exes may have had for him for ulterior motives, he wasn't forcing them into the relationship. He wasn't manipulating them.
The only one who was manipulating here was his beloved friend with whom he shared an already established relationship of trust and friendship, who betrayed their friendship and used his affection in order to pressure him into uncomfortable situations and sexual interactions that he wasn't interested in. The uke is nothing but a slightly toned down version of classic yandere girl archetype in male form or your classic seme bitch archetype in uke form. The only difference being that instead of the classic tropes like assault, kidnapping, threatening their near and dear ones or their livelihood, forcing them into debt or taking advantage of their bad circumstances and the like, he's just using the friendship that his sexual interest feels towards him in order to force him into a sexual relationship.
This is just a porno, so I guess you can't go moralizing here when people are getting off to far far worse plotlines, like pedophilia and incest. But, if you are in the "seme" guy's situation in real life, you should get away from such a "friend", no matter how close, as soon as possible. It will be painful and difficult to not just lose what you felt was a close friendship but also to realise that the friend you cared for and trusted all this time was never really a friend at all, but don't be like this guy. Don't just submit to toxic behaviour and manipulation just to preserve a social relationship if you don't feel like you have the upper hand there. You owe the duty of care and protection to yourself, before you do to anyone else. Yes, losing old friendships can be tough, and you may feel the urge to stick around in the hope that you can guide the problematic individual out of their problematic attitude towards you and get back your old relationship but do not do that. Just like with sexual relationships, losing a friendship is not the end of the world. There's always other fish in the sea.
And it's not your responsibility to fix them, you're not their mom. Even if you do still care for them, and want to help them out, do so from a safe place away from them by talking to uninvolved third parties and their family, their actual mom, and asking them to get your ex- friend the help they need. Don't stick around just because you want to troubleshoot. You don't try to rescue a bad situation when you are not in control or aren't prepared for it. You get yourself to a safe place and then leave the rest to people who are better equipped to handle it than you are.
Uke may be a psychotic manipulator but seme is his own type of trash LMAO since for him gf = sex and he doesn't actually seem to emotionally care about his exes